1. Field of the Invention
The present invention relates to a color image forming apparatus that forms a color image using an electrophotographic method, and a control method therefor.
2. Description of the Related Art
Recently, image forming apparatus, such as copying machines printers, are demanded to form high-quality images, at high speed with high accuracy. Particularly, for color image forming apparatuses, as a means for increasing the speed, there have been employed, for example, a tandem method in which for a plurality of colors (the four colors of yellow, magenta, cyan, and black), respective image forming units are provided for sequential image forming operations. Each image forming unit performs image formation by an electrophotographic process including electrostatic charging, exposure, development of an image, and transfer of the image to a sheet.
The tandem-type color image forming apparatus is advantageous in that it can attain high printing speed, but is disadvantageous in that it is difficult to reduce color misregistration when superposing a plurality of color images one upon another. Even if the color misregistration correction is carried out immediately after the image forming apparatus is installed, misregistration is produced as time elapses, and even if the color misregistration correction is carried out immediately before the image forming apparatus is used, subtle displacement occurs due to a change in temperature. Such displacement causes color misregistration in a color image formed by superposition of color images.
To solve the problem, there have recently been proposed lots of techniques for preventing such color misregistration. For example, a position-detecting patch is formed on a transfer belt for detecting color misregistration, and the patch is detected by a CCD line sensor to thereby detect color misregistration of each color (see e.g. Japanese Patent Laid-Open Publication No. H06-18796). Further, two or more position-detecting patches are detected by optical sensors to thereby detect color misregistration of each color (see e.g. Japanese Patent Laid-Open Publication No. H06-118735).
The means for detection of position-detecting patches to correct color misregistration executes the detection every certain (predetermined) number of sheets or at certain (predetermined) time intervals, and position detecting data obtained by reading the patches of all the colors are averaged for use in color misregistration correction. The position-detecting patches are basically formed by patches with maximum densities, and if the large number of patches are required, larger amounts of toners (developing agents) are consumed, and further, an increased burden is laid on cleaning members for collecting the toners since an image of the patches is not transferred to a sheet.
Now, a technique has been proposed which uses density sensors for detecting toner densities also as position-detecting sensors for detecting color misregistration, with a view to reducing the amount of toner consumed for color misregistration correction (see e.g. Japanese Patent Laid-Open Publication No. 2004-109682).
However, the proposed technique of using the density sensors for detecting toner densities also as the position-detecting sensors for detecting color misregistration suffers from the problem that the density sensors cannot provide sufficient accuracy in detecting color misregistration.